1. Technical Field
This invention relates generally to mobile cellular telecommunications, and more particularly, to a method of utilizing existing signaling protocols, such as the American National Standards Institute (ANSI)-41 protocol, to implement more efficient handoff back operations.
2. History of Related Art
Within a telecommunications network that supports Mobile Station (MS) roaming, several methods have been devised to transfer communications for active call connections maintained by any one MS as it moves away from one serving switch toward another. The American National Standards Institute (ANSI)-41 D Standard includes several procedures and messages to implement such inter-system (inter-exchange) handoff forward operations (i.e., transferring the active call connection of a MS from the serving switch to a target switch which is not currently trunk-connected in the call), and handoff back operations (i.e., transferring the active call connection from the serving switch to a target switch which is already trunk-connected within the call path). While several scenarios are offered for handling handoff forward operations with a tandem switch, only two scenarios are available when a handoff back operation must be accomplished. Further, the handoff back call connection operation is only valid for use when the target switch is connected directly to the serving switch (i.e., there is a direct trunk connection therebetween). Thus, for example, the handoff back message cannot be used to transfer a call connection from a serving Mobile Switching Center (MSC) to an anchor MSC using the anchor MSC as the target switch If there is a tandem MSC located along the call path between the anchor MSC and the serving MSC.
While it is possible to invoke the handoff back operation under such circumstances (i.e., where multiple switches are trunk-connected along a call path, and the target switch is not immediately trunk-connectable to the serving switch), the result will be that trunks between the various switches (connected before handoff) will not be released after the handoff operation occurs. Thus, network resources are wasted.
Another problem occurs when a HandoffToThird message is received by a tandem or anchor switch which does not support a handoff operation with path minimization. The result is a RETURN ERROR or REJECT response sent back to the requesting switch. Alternatively, no response at all may be received by the requesting switch. When this occurs, the serving switch must send a FacilitiesDirective message back along the call path, past the tandem switch, so that the handoff can occur. However, once again, previously connected trunks along the call path will not be released, and network resources will be wasted.
These scenarios are illustrated in FIGS. 1, 2, 3, and 4. A successful sequence of handoff operations in the prior art can be seen in the network signaling and nodal operation diagram of FIG. 1. In this case, a successful handoff back with tandem using the HandoffToThird message and path minimization are shown within a telecommunications network 15 including a telephone 40, an anchor MSC 60, a tandem1 MSC 70, a serving MSC 90, and a MS 50 are shown. There is an active call connection between the telephone 40 and MS 50, using switches 60, 70, and 90, which are connected by trunks 120 and 130. The telephone 40 (which may be a wireline telephone or another MS) is connected to the switches 60, 70, and 90 by way of a residential line connection or wireless network interface 110, and the MS 50 is connected to the serving MSC using a wireless network interface 140.
If the serving MSC 90 operates to determine that a handoff operation to a nearby candidate MSC, such as the anchor MSC 60, is appropriate, the serving MSC 90 may send a HandoffMeasurementRequest message 150 to the anchor MSC 60. In response, the candidate MSC 60 performs location measurements according to its internal programming algorithms, and returns the results to the serving MSC 90 in the form of a HandoffMeasurementRequest response 160. Based on the response 160 content, the serving MSC 90 determines that the call should be handed off to the anchor MSC 60, which is now also considered the target MSC 60. The anchor MSC 60 will be directed to receive the MS 50 using the handoff back call connection with path minimization operation, and the MS 50 will be moved to the designated channel 107 of the anchor MSC 60. Directing the anchor MSC 60 to receive the MS 50 involves sending a HandoffToThird message 290 from the serving MSC 90 to the tandem1 MSC 70, which in turn directs the tandem1 MSG 70 to perform an intersystem handoff task with path minimization; sending a HandoffToThird message 320 from the tandem1 MSC 70 to the anchor MSC 60, which results in directing the anchor MSC 60 to perform task minimization and verifies that the designated channel 107 of the anchor MSC 60 is available to support the MS 50; and sending a HandoffToThird response along the call path from the anchor MSC 60 to the serving MSC 90 (steps 330 and 340).
Moving the MS 50 to the designated channel 107 of the anchor MSC 60 requires sending a Mobile Handoff Order 190 from the serving MSC 90 to the MS 50 and receiving the MS 50 on the designated channel 107 at arrival step 200.
Directing the tandem1 MSC 70 and serving MSC 90 to release the first and second inter-MSC trunks 120 and 130, and marking the first and second inter-MSC trunks 120 and 130 as idle requires sending a FacilitiesRelease message along the call path from the anchor MSC 60 to the serving MSC 90 (i.e., steps 220 and 230), sending a FacilitiesRelease response 260 along the call path from the serving MSC 90 to the tandem1 MSC 70, and sending a FacilitiesRelease response 270 along the call path from the tandem1 MSC 70 to the anchor MSC 60. At this point, the call path includes only the telephone line or wireless network interface 110, the anchor. MSC 60, and the wireless network interface 280 between the telephone 40 and the MS 50. No unused inter-MSC trunks 120, 130 are left connected, and network resources are conserved.
FIG. 2 is a network signaling and nodal operation diagram illustrating prior art handoff back operations, wherein the telecommunications network 10 includes a telephone 40, an anchor MSC 60, a tandem1 MSC 70, tandem2 MSC 80, serving MSC 90, and a MS 50. There is an active call connection between the telephone 40 and MS 50, using a series of switches 100, which are connected by trunks 120, 130, and 135. The telephone 40 (which may be a wireline telephone or another MS) is connected to the switches by way of a residential line connection or wireless network interface 110 and the MS 50 is connected to the serving MSC using a wireless network interface 140.
If the serving MSC 90 operates to determine whether a handoff operation to an adjacent candidate MSC, such as the tandem2 MSC 80, is appropriate, the serving MSC 90 may send a HandoffMeasurementRequest message 150 to the tandem2 MSC 80. In response, the candidate MSC 80 performs location measurements according to its internal programming algorithms, and returns the results to the serving MSC 90 in the form of a HandoffMeasurementRequest response 160. Based on the response 160 content, the serving MSC 90 determines that the call should be handed off to the tandem2 MSC 80, which is now also considered the target MSC 80. Further, the serving MSC 90 has determined that the tandem2 MSC 80 is already trunk-connected along the call path.
At this point, a HandoffBack message 163 is sent from the serving MSC 90 to the target MSC 80, which directs the target MSC 80 to initiate a handoff back task. If a voice channel on the target MSC 80 is available, a HandoffBack response 167 is returned to the serving MSC 90, which allows the handoff operation to proceed.
The serving MSC 90, upon receipt of the HandoffBack response 167, sends a Handoff Order message 190 to the served MS 50. This action directs the MS 50 to move to the available voice channel on the target MSC 80. Upon arrival 200 of the MS 50 on the voice channel of the target MSC 80, a FacilitiesRelease message 202 is sent from the target MSC 80 to the serving MSC 90. This indicates that the handoff operation has been successful and that the facilities used by the serving MSC 90 are no longer needed. The serving MSC 90, in turn, sends a FacilitiesRelease response 204 to the target MSC 80, releasing the trunk connection 135 and marking the inter-MSC trunk 135 as idle. The target MSC 80, in turn, marks the inter-MSC trunk 135 as idle and the handoff back process is complete. The resulting call connection between the telephone 40 and the MS 50 comprises the series of switches 100 connected by trunks 120 and 130. The telephone 40 is connected to the switches 100 by way of line connection or wireless interface 110, and the MS 50 is connected to the new serving MSC 80 using the wireless network interface 137.
The scenario 20 just described, is a classic prior art handoff back operation from a serving switch 100 to a tandem switch 100, where the tandem switch 100 is trunk-connected to the serving switch 100 along the call path. In this case, no network resources are wasted, and the ANSI-41 protocol operation functions in an efficient manner.
Turning now to scenario 30 of FIG. 3, the prior art handoff back operation to a tandem switch 100 (i.e., tandem1 MSC 70), which is in the call path, but is not trunk-connected to the serving MSG 90 and path minimization is not supported, is illustrated. In this case, as will be described, a handoff back operation results in wasted network resources.
As noted above, an active call connection between the telephone 40 and the Mobile Station 50 exists, making use of trunk connections 120, 130, and 135. A HandoffMeasurementRequest message 150 is sent from the serving MSC 90 to the tandem1 MSC 70. The HandoffMeasurementRequest response 160 is returned to the serving MSC 90, where it is determined that a handoff back operation from the serving MSC 90 to the tandem1 MSC 70 is desirable.
A FacilitiesDirective message 165 is then sent from the serving MSC 90 to the tandem1 MSC 70, which results in a FacilitiesDirective response 169 from the tandem1 MSC 70 to the serving MSC 90. The Handoff Order 190 is sent to MS 50, and the MS 50 arrives 200 on the available channel of the tandem1 MSC 70. The tandem1 MSC 70 sends a Mobile Station On Channel message 206 to the initiator of the handoff back task (i.e., the serving MSC 90), informing the serving MSC 90 that the tandem1 MSC 70 has completed the handoff back task. Unfortunately, at this point, all of the trunks 120, 130, and 135 are still connected. Moreover, a new trunk 138, has also been connected between the tandem1 MSC 70 and the serving MSC 90. The resulting call connection between the telephone 40 and the MS 50 comprises the series of switches 100 connected by trunks 120, 130, 135, and 138. The telephone 40 is connected to the switches 100 by way of line connection or wireless interface 110, and the MS 50 is connected to the new serving MSC 70 using the wireless network interface 139. Network resources are wasted in this case, because the most efficient connection between the telephone 40 and the MS 50 should require only the trunk connection 120, such that the network resources tandem2 MSC 80, serving MSC 90, and trunks 130, 135, and 138 can be released.
Referring now to FIG. 4, scenario 35 illustrates what can happen in prior art communications when a HandoffToThird message is sent to a switch which is incapable of accepting the message for execution. The same result may occur if no response is received to the message due to congestion within the network 15, or for other reasons recognized by those skilled in the art.
In this case, a HandoffMeasurementRequest message 150 is sent from the serving MSC 90 to the tandem1 MSC 70 (the desired target switch 100), which elicits a HandoffMeasurementRequest response 160 from the tandem1 MSC 70 to the serving MSC 90. The serving MSC 90, in turn, sends a HandoffToThird message 290 to the tandem2 MSC 80, which is incapable of processing the message 290. As noted above, there may be no response because the tandem2 MSC 80 does not support the HandoffToThird command, or possibly, because the network 15 is congested. In any event, an error 300 occurs, and the serving MSC 90 receives a response such as RETURN ERROR, REJECT, or NO RESPONSE (i.e., no response at all is received).
Since the tandem2 MSC 80 is incapable of supporting a handoff operation with path minimization (as evidenced by the error message), a FacilitiesDirective message 165 is instead sent from the serving MSC 90 to the tandem1 MSC 70, or target MSC 70. Once the tandem1 MSC 70 verifies that a designated channel is available for use by the MS 50, a FacilitiesDirective response 169 is sent from the tandem1 MSC 70 to the serving MSC 90. This results in a Handoff Order message 190 being sent from the serving MSC 90 to the MS 50 directing movement of the MS 50 onto the designated channel at the tandem1 MSC 70. After arrival 200 of the MS 50 on the designated channel at the tandem1 MSC 70, the tandem1 MSC 70 will send a Mobile Station On Channel message 206 to the serving MSC 90. Once again, the problem of excessive trunks remaining in use exists. In this case, the original trunks 120, 130, and 135 are still connected, with the additional trunk 138 also having been established as a result of the handoff operation. Network resources in this situation are wasted in this case because the most efficient connection between the telephone 40 and the MS 50 should require only the trunk connection 120, such that the network resources tandem2 MSC 80, serving MSC 90, and trunks 130, 135, and 138 can be released.
Therefore, what is needed is a method to support a handoff back call connection operation within a telecommunications network, including multiple, connected switches, that enables the release of unused trunks along the call path when the target switch is already active within the call path, and not directly connected to the serving switch. Such a method should be usable within the current ANSI-41 D Standard, and also be backward-compatible with previous versions of the standard. Such a method should require no changes in currently-available network switch hardware.
The method of the present invention operates to enable handoff back operations within a telecommunications network during two different call connection scenarios. First, when path minimization is not possible, and the handoff occurs between a serving switch and a target switch, wherein the target switch is already connected as an anchor or a tandem within the call path, and not directly trunk-connected to the serving switch. Second, where the call path is the same as in the first scenario, and path minimization may be possible, but it is determined that one of the switches in the call path does not support path minimization operations. In each case, the target MSC is a switch in the call path that has no direct trunk connection to the switch currently serving the MS, which is the circumstance leading to inefficiencies and wasted resources using current ANSI-41 D handoff back operations.
When path minimization is not supported among the switches of the active call path, the method supporting mobile station handoff back call connection operations within a telecommunications network includes the steps of determining that a handoff back call connection operation to a target MSC should occur, verifying that a designated channel on the target MSC is available to support the Mobile Station, moving the Mobile Station to the designated channel of the target MSC, directing the switches along the call path that are no longer needed to release any unnecessary inter-MSC trunks, and marking the released inter-MSC trunks as idle.
If path minimization operations may be possible, but are found to not be supported by at least one switch within the call path, the method supporting mobile station handoff back call connection operations within a telecommunications network comprises the steps of determining that a handoff back call connection operation to a target MSC should occur, determining that the target MSC will not support path minimization operations, verifying that a designated channel on the target MSC is available to support the Mobile Station, moving the Mobile Station to the designated channel of the target MSC, directing the switches along the call path that are no longer needed to release any unnecessary inter-MSC trunks, and marking the released inter-MSC trunks as idle. The step of determining that the target MSC (or any tandem along the call path) will not support a path minimization operation may be accomplished by sending a HandoffToThird message from the serving MSC to the MSC which will not support path minimization (i.e., a non-capable MSC), which directs the non-capable MSC to perform path minimization, and receiving a response from the non-capable MSC at the serving MSC, which is a RETURN ERROR, REJECT, or NO RESPONSE (meaning that no response at all is received within the timeout period of the service MSC or switch message reception timer).